As is well known, multipath interference is caused when two or more signal rays of an original transmitted signal converge upon a receiving antenna of a receiver at significantly different times. This misalignment or superposition of signals, which are replicas of the original signal, may cause distortion in audio recovered from the signals. Distortion caused by the multipath interference may be attributable to long delay (e.g., greater than five microseconds between signals) multipath interference or short delay (e.g., less than five microseconds between signals) multipath interference.
In a typical urban environment, RF signals experience changes in amplitude and phase due to short delay multipath. This amplitude and phase shift may result in broadband signal fades of up to 40 dB as the receiver and its associated motor vehicle change locations (see FIG. 1, signal 102 of graph 100). At typical highway speeds, signal fluctuation rates in the range of 100 to 1 kHz may occur. In general, long delay multipath (or frequency selective multipath) is found in areas where reflectors are greater than four to five miles away. Typically, long delay multipath occurs in cities with large buildings and in mountainous regions.
Generally, long and short delay multipath coexists and creates frequency selective fading and broadband fading, simultaneously (see FIG. 2, RF level signal 204 and FM demodulator output (MPX) signal 202 of graph 200, which depicts exemplary audio distortion attributable to long delay multipath).
For example, as is shown in FIG. 2, the signal 202 may contain a 1 kHz tone with a 75 kHz deviation. In such a situation, a reflected signal may have an amplitude of, for example, 0.9 units while a direct signal has, for example, an amplitude of 1 unit. In the case where the time delay of the reflected signal is about 30 microseconds, the distortion attributable to the time delay may be on the order of approximately twelve percent.
In various receiver systems, antenna diversity has been implemented in conjunction with an FM receiver to reduce degraded reception performance caused by multipath interference. Typically, antenna diversity has been accomplished through the use of at least two uncorrelated antennas. Prior art antenna diversity reception for mobile communication systems has been achieved by implementation of a number of different systems. For example, antenna diversity has been accomplished with equal gain combiner (EGC) systems, maximal ratio combiner (MRC) systems and antenna diversity systems, such as the adaptive reception system (ARS) disclosed in U.S. Pat. No. 5,517,686, the disclosure of which is hereby incorporated herein by reference in its entirety.
In general, EGC systems and MRC systems outperform switched antenna diversity systems. However, EGC and MRC systems tend to be more expensive to implement, as they require multiple analog front-ends. Typically, EGC and MRC systems attempt to optimize certain characteristics of a received signal in order to minimize the number of fades or maximize the desired signal.
In a switched antenna diversity system, only one antenna is utilized for reception at any instant in time and, thus, the non-selected antennas do not contribute to the demodulated signal. In contrast, EGC and MRC systems utilize signals from all antennas through a variety of combining techniques.
Constant modulus algorithm (CMA) systems have been implemented extensively in digital broadcasting. In general, combining signals from multiple antennas advantageously provides antenna directionality, which allows interferers (i.e., multipath rays) to be suppressed by creating a null in the antenna pattern in the direction of the multipath interferer. In such systems, the method for determining the combining weights varies, depending upon the application. In the case of an FM receiver implemented in a mobile application, the combining of the signals is particularly challenging, as an implemented algorithm must adapt as the vehicle moves. In a usual case, adaptation is blind, i.e., there is no cooperation between the transmitter and receiver, and the received signal is an analog signal. As such, signal imperfections are difficult to mask.
However, FM modulated signals initially have a constant modulus (amplitude), which provides prior knowledge about the transmitted signal (see FIG. 3, signal 302 of graph 300). Unfortunately, in a multipath reception environment, a received signal no longer possesses this constant modulus property, as a result of flat or frequency selective fading, and, as such, the complex baseband signal trajectory is no longer a circle but may take the form of a complicated spiro-graph (see FIG. 4, graph 400, with spiro-graph first antenna signal 402, and FIG. 5, graph 500, with spiro-graph second antenna signal 502). Fortunately, in systems that combine antenna signals, the signal amplitude information can be utilized to determine combining weights for the signals from the different antennas.
In general, for narrowband modulation, signal combining may be accomplished by adjusting the amplitudes and phases of the individual antenna signals, prior to summation, to optimize appropriate receiver metrics. The gain and phase applied to each antenna signal is equivalent to complex weighting in the complex baseband model and are referred to as antenna or combining weights. The weighted summation of the individual antenna signals results in an overall reception pattern. The combining method determines the weights which optimize certain receiver metrics. In practice, an iterative solution is generally used to solve for the optimal weights and the system is said to ‘adapt’.
The MRC method attempts to maximize some measure of signal-to-noise ratio (SNR) (often called CNR). The CMA method attempts to minimize variation of the received signal amplitude. The CMA method is known to steer the antenna pattern so as to place nulls in the direction of the interfering signals (multipath reflections in this case). The combined antenna pattern varies with the frequency of the received signal so for an FM modulated signal the weights must be adapted rapidly such that the multipath reflection is nulled as it moves in frequency due to the modulation. Because the signal modulation is normally not present in the amplitude of an FM signal, the CMA adaptation bandwidth can extend into the modulation bandwidth such that the adaptation can be fast enough to maintain the necessary null direction as the frequency of the received signal changes due to the modulation.
In general, the CMA method performs well for FM multipath, especially in strong signal conditions where the received distortions are dominated by multipath interference, as opposed to noise. However, as the received signal becomes weak, the amplitude becomes corrupted by noise and the constant modulus assumption of the received signal begins to fail and the CMA performance correspondingly degrades. In addition, the CMA can ‘lock’ to spurs in the received signal that become unmasked as the desired signal becomes weak. In contrast, the MRC adaptation is influenced by the signal modulation and, thus, the adaptation bandwidth is limited. However, the MRC assumptions remain valid even for weak received signals. Therefore, while the CMA tends to outperform the MRC for strong signals, the CMA may have difficulty with weak signals and, in this case, the MRC tends to yield better and more consistent performance.
With reference to FIG. 6, an exemplary block diagram 600 of an MRC algorithm includes function blocks 602, 604 and 606, which are implemented in software. With reference to FIG. 7, an exemplary block diagram 700 of a constant modulus algorithm (CMA) includes function blocks 702, 704 and 706, which are implemented in software.
The CMA is a relatively efficient algorithm that allows for adaptation, based on amplitude variation of known constant amplitude signals. In a two antenna case with a single interferer, it is usually possible to position a null in the direction of the multipath interferer (i.e., in the long delay multipath). For an incident plane wave, only the phase of the received signals differ between the two antennas and, thus, combining weights can be found such that the signals cancel in the combined output. While this solution does, in fact, satisfy the constant modulus constraint, it does not guarantee that a system implementing a CMA will adapt to the solution and, in general, the solution may not be unique. For example, a null can be placed in the direction of the direct path and still satisfy the CMA constraint. While this may be acceptable, as noted above, the assumption of constant modulus degrades as the received signal becomes weak, as noise violates the constant modulus property. Furthermore, the wide adaptation bandwidth, which makes the CMA effective in long delay multipath, results in noisy adaptation for weak signals.
What is needed is an improved technique for reducing multipath distortion in a mobile FM receiver having multiple antennas.